1. Technical Field
The invention relates generally to methods and apparatus for measuring the effects of high-intensity, electromagnetic radiation, e. g. , deep ultraviolet radiation, on the optical properties of an optical sample. More specifically, the invention relates to a method and an apparatus for conducting accelerated damage testing of an optical sample.
2. Background Art
One of the important consequences of exposing optical materials, including anti-reflection and high-reflection optical coatings, to high-intensity, electromagnetic radiation is possible degradation of their optical properties. For example, the response of high-purity, fused-silica glass to prolonged exposure to deep ultraviolet (UV) radiation such as 193 nm or 248 nm radiation is gradual monotonic development of absorption at the wavelength of the exposure beam. Increased absorption in the fused silica has an adverse effect on the transmission capabilities of the fused silica. Degradation of optical coatings exposed to deep UV radiation is also well known. Because optical materials (and optical coatings) can degrade over time under exposure to high-intensity, electromagnetic radiation, it is useful to be able to characterize the performance of the optical material under real-life conditions. Such characterization would be useful in determining the suitability of the optical material for a specific application and the useful life of the optical material.
The real-life performance of an optical sample under prolonged exposure to high-intensity, electromagnetic radiation can be estimated by applying doses of laser pulses to the optical sample and then measuring changes in a selected optical property (or properties) of the optical sample. Typically, the optical property of interest is absorption coefficient because it provides a direct measure of the transmission capabilities of the optical sample. For anti-reflective and high-reflective optical samples, the optical property of interest is typically reflectivity.
Currently, the damage testing requirements for a high-purity, fused-silica glass used in fabricating microlithography stepper and scanner lenses are specified as 1011 pulses of a 193 nm ArF excimer laser using an output energy intensity of 0. 1 to 0. 5 mJ/cm2. At a pulse repetition rate of 400 Hz, it would take approximately eight years to test the glass at this low output energy intensity, which simulates the actual product dosage. It may be possible to fit the damage testing into a practical timeframe by increasing the repetition rate of the laser pulse. The repetition rate of a laser pulse is, however, limited by the capabilities of the laser. For example, the repetition rate of a commercially available ArF laser is currently limited to 400 Hz. Alternatively, the damage testing may be accelerated by using a laser having a higher output energy-intensity. It should noted, however, that the use of a higher output energy intensity may not produce glass behavior that is representative of the lower energy intensity that the product is exposed to in real-life operations. Damage thresholds can also be easily exceeded if the output energy-intensity of the laser pulse is too high. If damage thresholds are exceeded, extrapolation of the test results to real-life performance can be very misleading.
In one aspect, the invention relates to an accelerated radiation damage testing method for an optical sample which comprises disposing the optical sample inside an optical cavity so as to form an optically stable resonator. The method further includes injecting a predetermined number of light pulses into the optical cavity at a selected wavelength and at spaced intervals, allowing each light pulse injected into the optical cavity to decay to a selected value, and determining a change in an optical property of the optical sample after the optical sample has been exposed to the predetermined number of light pulses. In one embodiment, determining a change in an optical property of the optical sample includes monitoring the decay rate of the light pulse. In another embodiment, the optical sample is positioned substantially in the middle of the optical cavity. In another embodiment, a spatial length of each light pulse is much smaller than a length of the optical cavity.
In another aspect, the invention relates to an accelerated radiation damage testing method for an optical sample which comprises disposing the optical sample at an output end of an optical cavity. The method further includes injecting a predetermined number of light pulses into an input end of the optical cavity at a selected wavelength and at spaced intervals and allowing the light pulse in the optical cavity to decay to a selected value. A train of pulses is transmitted through the output end of the optical cavity and is focused on the optical sample. The method includes determining a change in an optical property of the optical sample after the optical sample has been exposed to a predetermined number of pulses.
In another aspect, the invention relates to an apparatus for conducting accelerated damage testing on an optical sample which comprises means for generating light pulses at a selected wavelength and at spaced intervals. The apparatus further includes an optically stable resonator for receiving the light pulses and producing a train of discrete pulses which are focused on the optical samples and means for monitoring the intensity of each light pulse.